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| Home > Publications database > Interface Functionalization of Magnetic Oxide Fe$_{3}$O$_{4}$/SrTiO$_{3}$ Heterostructures |
| Book/Dissertation / PhD Thesis | FZJ-2021-01792 |
2021
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-535-2
Please use a persistent id in citations: http://hdl.handle.net/2128/27670
Abstract: Oxide heterostructures possess a wide range of electrical and magnetic properties, mainlyvia interactions across their interfaces. The prospect of designing and controlling the magnetic properties at the atomic scale of oxide hetero interfaces is one of the major challenges. In this context, merging transition-metal oxides into heterostructures is very promising, owing to their many remarkable properties, such as emerging conductivities, magnetism or ferroelectricity. Furthermore, iron oxides including FeO, Fe$_{3}$O$_{4}$ and Fe$_{2}$O$_{3}$ polymorphs ($\alpha$Fe$_{2}$O$_{3}$, $\gamma$Fe$_{2}$O$_{3}$...) with a multitude of electric and magnetic functionalities are interesting for many magnetic applications and heterogeneous catalysis. Controlling the oxide interfaces additionally strengthens the manufacturing of functional devices. Therefore, our primary goal is understanding, controlling and tuning the interface properties. For this purpose, we demonstrate the emergence and control of magnetic interfaces between magnetite Fe$_{3}$O$_{4}$, a ferrimagnetic half-metal, and SrTiO$_{3}$, a transparent nonmagnetic insulator which is considered the bedrock of oxide-based electronics. The Verwey transition (T$_{V}$ ) is found to persist from bulk-like down to ultrathin Fe$_{3}$O$_{4}$ films, decreasing from 117±4K (38nm) to 25±4K (2nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent HAXPES and XMCD techniques. We prove that the SrTiO$_{3}$ substrates induce both strain and interface oxidation. The substrate-induced strain causes the easy axis to switch to [100]. Furthermore, we observe a reduction of Fe2$^{+}$ ions with decreasing film thickness, accompanied by an increase of Fe3$^{+}$ ions in both tetrahedral and octahedral sites, and conclude on the formation of a magnetically active ferrimagnetic 2u.c. $\gamma$Fe$_{2}$O$_{3}$ intralayer. To manipulate the interfacial magnetic phase, a post-annealing process is conducted which causes the reduction of the $\gamma$Fe$_{2}$O$_{3}$ that finally leads to stoichiometric and ferrimagnetic Fe$_{3}$O$_{4}$/SrTiO$_{3}$ (001) heterointerfaces. We demonstrate the thermally induced phase transformations between Fe$_{3}$O$_{4}$, $\gamma$Fe$_{2}$O$_{3}$ and FeO ultrathin iron oxide films, which are part of all-oxide heterostructures, and present a comprehensive thermodynamic analysis of the emerging interfacial redox processes through active redox reactions across three relevant interfaces, i.e. (1) the outside atmosphere/Fe$_{x}$O$_{y}$ film interface, (2) the interface between Fe$_{x}$O$_{y}$/Fe$_{x}$O$_{y}$ intralayers and (3) the Fe$_{x}$O$_{y}$/oxide substrate interface. We thereby reveal the essential – but mostly underrated – role of oxide substrates, which can completely alter the standard FexOy temperature-pressure phase diagram as an additional oxygen supplier or scavenger. We introduce an adjusted phase diagram specifically for Fe$_{x}$O$_{y}$/ Nb:SrTiO$_{3}$ and Fe$_{x}$O$_{y}$ / YSZ heterostructures based on a total effective oxygen activity. Our study goes beyond the current functionalization of oxide heterostructures and their phase transitions. This novel approach opens up the route towards reversible tuning of the physical functionalities, thus, a future integration of Fe$_{3}$O$_{4}$/SrTiO$_{3}$ heterostructures into resistive and magnetic switching devices.
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